Dark trace cathode-ray tube and method of manufacture



G. LEVY July 17, 1956 DARK TRACE CATHODEl-RAY TUBE AND METHOD OFMANUFACTURE Filed March 25, 1954 Y. R my w my 2 Z V v 2 u y a nitedStates Patent Oflice 2,755,404 P atented July 17, 1 95s DARK TRACECATHODE=RAY TUBE AND NETHOD OF MANUFACTURE Gustave Levy, East Orange, N.J., assignor to National Union Electric Corporation, a corporation ofDelaware Application March 25, 1954, Serial No. 418,735

15 Claims. (Cl. 313-91) This invention relates to so-called dark tracetubes employing a cathode-ray responsive screen of scotophor material,and the like.

As is known in the art, the dark trace type of cathoderay tube comprisesan electron gun for developing a defiectable electron beam for scanninga special screen wherein the light-transducing material comprises ascotophor or mixture of scotophors. Such scotophors may consist of anyof the alkali metal halides, preferably potassium chloride. One of thelimitations on the use of such tubes in certain fields of use is thedifficulty or slowness of erasing the dark trace record. It has beenknown for some time that such dark trace records can be erased by heat,by ultraviolet light, or by the electron bombardment. However, each ofthese known ways of erasure has certain defects which it is the objectof this invention to alleviate.

Accordingly, one of the principal objects of this invention is toprovide an improved structure of dark trace or scotophor screen whosedark trace record can be more rapidly erased.

Anotherobject is to provide a novel dark trace screen which produces ahigh degree of contrast in the dark trace record and which is capable ofrelatively rapid erasure.

Another principal object is to improve the contrast, while at the sametime increasing the useful life and permitting back lighting of ascotophor screen, and also simplifying the power supply required for theerasure energy.

A feature of the invention relates to a dark trace cathode-ray tubehaving a scotophor screen which is attached to a sheet of lighttranslucent material of extreme thinness, the scotophor being coatedwith a layer of microcrystalline aluminum of controlled thinness. Thisaluminum layer is associated with a heat radiation source rich ininfra-red radiation so that the aluminum acts as a wavelength converterfor the radiation heat energy and thereby more efilciently erases thedark trace record in the scotophor.

A further feature relates to the novel method of preparing scotophorscreens for dark trace cathode-ray tubes and the like.

A still further feature relates to the novel organization, arrangementand relative location, and preparation of parts which cooperate toprovide an improved dark trace cathode-ray tube having rapid erasurecharacteristics and high contrast.

In the drawing,

Fig. 1 is a longitudinal plan view of a typical dark trace tubeembodying the invention.

Fig. 2 is a greatly magnified view of a section of the dark trace screenof the tube of Fig. 1.

Fig. 3 is a schematic view of one typical apparatus for applying theheat converting coating to the scotophor.

In the drawing, which is merely by way of example, there is shown inschematic form an evacuated bulb of glass having suitably mounted withinthe neck portion thereof any Well-known electron gun 11 for developingan electron beam of high velocity electrons e. g., of the order of10,000 electron volts. This gun includes for example the heatableelectron-emitting cathode 12 with its internal heater element 13; acentrally apertured metal disc 14, which may constitute a control gridfor controlling the brightness or intensity of the electron beam 15; afirst beam-accelerating and focussing anode 16; a second and highervoltage anode 17; and the conventional coordinate beam-deflectingelements which may be the usual electrostatic deflector plates 18, 19.

Mounted within the bulb 10 adjacent the fiat end thereof is the screen20, constituted of an extremely thin sheet of mica or glass 21, forexample .0009 inch to .0014 inch, which carries the coating 22 ofscotophor material such as an alkali metal halide, preferably, althoughnot necessarily, a potassium chloride crystal layer. This alkali metalhalide layer 22 is covered with a coating 23 of aluminum or similarlight-weight metal or critically controlled thinness. Mounted in frontof the screen 20 on the side facing the electron gun 11 is a source 24of radiant heat rich in infrared for uniformly heating the entiresurface of the layer 23 by radiant energy. In accordance with theinvention, the source 24 is a fine wire tungsten filament which can beconnected to a suitable current supply source (not shown) to heat thefilament to a temperature at which it acts as an efiicient source ofinfra-red radiant energy. For example, it is an efficient radiationsource of wavelengths shorter than 25 X 10- cm. The scotophor layer 22transmits all wavelengths shorter than 25 X l0 cm. Therefore, thescotophor 22 by itself does not absorb sufficient energy in that rangeto effect rapid erasure of the dark trace record in the scotophor. Inorder that the erasure filament 24 shall not interfere with therecording, it may be in the form of a very fine wire annular or toroidalwinding which is outside the maximum deflection angle of the beam 15. Orit may be a fine zig-Zag filament adjacent the screen 20.

As is well-known, when the cathode-ray beam is of sufiicient intensity,usually expressed kilovolts, e. g., 8 to 14 kv., as the beam scans thescotophor layer 20, it develops so-called opacity centers or F-centersin accordance with the instantaneous intensity of the beam as modulatedby the signal voltages applied to grid 14. I have found that by usingthe construction of screen as above described and using an erasuresource 24 which is an efficient infra-red radiator, it is possible toerase the previously developed opacity centers in a relatively shorttime by applying heating current for a corresponding short time to thefilament 24 and without subjecting the scotophor screen to electronbombardment for erasure purposes.

In order to effect the proper and rapid erasure, it is necessary tocondition the screen so that it will absorb the radiant infra-red energyfrom source 24. The KCl material 22 is preferably deposited in a vacuumon the mica sheet 21. Likewise, the aluminum coating 23 is preferablydeposited by vaporization in a vacuum. I have found that this aluminumlayer should be of sufiicient thinness so as to be transparent to theelectrons in the beam 15, and also of suificient thinness so that thescreen can be viewed from the front of the screen, while the screen isbeing illuminated from the rear, for example by a conventional lightsource 25 and a suitable optical system 26.

The thickness of the aluminum layer 23 is critical in that if it is toothick it will not properly pass the electron beam 15 to the scotophormaterial 22; on the other hand, it must be sufliciently thick to enableit to absorb suificient energy from the infra-red radiator 24 to erasethe dark trace record in the scotophor material within the requiredshort period of time.

Furthermore, since the scotophor material is in crystalline form itpresents" by itself a somewhat uneven surface upon which the aluminummust be evaporated. Advan:

tage is taken of this fact according to the invention to utilize thephenomena of anomalous dispersion at the surface of the aluminum layerto increase the absorption of heat from the infra-red filament source24. There is evidence to show that the dielectric constant B, and thespecific conductivity 5 of dielectrics and conductors vary incomplicated ways with frequency. Since the scotophor surface 22 is not asmoothly reflecting surface it is possible that with extremely thinaluminum coating, certain minute areas of the scotophor screen may notbe covered with aluminum so that the surface of the screen struck by thebeam at these minute points will exhibit anomalous dispersion. However,rather than being a disadvantage, this dispersion is an advantageaccording to the invention. The phenomenon of change of index ofrefraction with frequency is called dispersion, and where this change issudden there is anomalous dispersion. In the neighborhood of a region ofanomalous dispersion there is energy absorption even where the surfaceof the material does not absorb the energy elsewhere. Advantage is takenalso of this anomalous dispersion phenomenon to decrease the erasuretime of the dark trace record. The variation of dielectric constant Efor a metal such as the aluminum coating 23 with frequency, is analogousto that of a dielectric. Therefore, by making the aluminum layer 23 ofthe desired critical thinness the absorption of energy in the scotophorresults not only from the anomalous dispersion but also the absorptionof energy accompanying the electron conductivity through the aluminumlayer.

I have found that one practical way for controlling the thinness of thealuminum layer 23 is to deposit the aluminum in successive steps in avacuum. Thus, the mica sheet 21 with its scotophor coat 22, can bemounted within the bell jar 27 which can be evacuated by any well-knownmeans. Located within the bell jaw 27, is a refractory fine wire metalfilament such as tantalum filament 28, to which had been previouslyattached one or more prefused aluminum pellets 29. It is thus possibleby applying a suitable heating current to the filament 27 in successivesteps to evaporate the aluminum 29 in corresponding steps upon thescotophor material 22. Suflicient time should elapse between each stepto allow the micro-crystalline deposit to cool down. In general, thetotal number of such successive steps can be determined by the physicalappearance of the scotophor screen. For example, on the first flashingof the aluminum pellets, the screen does not materially change itsnormal dark grayish color. After a few successive fiashings, the screenassumes a grayish appearance until after a number of steps, for example,five or six, the screen becomes whitened. This is an indication that thedesired thickness of aluminum has been applied to the scotophor. Thiswhitening of the screen not only has the desirable feature of indicatingthe end point of the aluminum evaporation steps but it also increasesthe contrast in the recorded image when it is being viewed from thefront end of the tube.

In one particular tube that was found to be satisfactory, it waspossible to erase the image on the scotophor screen completely within 12seconds by applying 300 watts of energy to the erasure filament 24. Therelation between the electrical wattage applied to erasure filament 24and the erasure time for the various successive evaporation steps inapplying the aluminum to the scotophor screen was as follows:

Watts ge on Filament 24 Erasure No. of Steps Time Since the aluminumlayer 23 is of extreme thinness it is not practical to measure it byconventional thickness measurements. Therefore, the above-noted methodof depositing it in successive steps and noting the change in appearancefrom dark gray to white appearance is a practical way of determining theultimate thickness of the aluminum coating.

Various changes and modifications may be made in the disclosedembodiment without departing from the spirit and scope of the invention.

What is claimed is:

1. A cathode-ray tube comprising in combination, an enclosing evacuatedenvelope, means to develop a beam of electrons, a screen upon which saidbeam impinges to make a record, said screen comprising a layer ofscotophor material which develops opacity centers when said beamimpinges thereon, and means to erase said centers, the last-mentionedmeans including a layer of aluminum on said scotophor material facingsaid beam-developing means, and an infra-red radiation generator withinsaid envelope between said beam-developing means and said screen, saidgenerator being especially designed to pro duce substantial radiation atwavelengths less than 25 l0 cm., said aluminum layer having a thinnesswhich is transparent to said beam but which develops heat by convertingsaid infra-red radiation into wavelengths which are substantiallyabsorbed by said scotophor material.

2. A cathode-ray tube according to claim 1, in which said scotophormaterial by itself transmits without substantial absorption infra-redwavelengths shorter than 25 X 10* cm., and said aluminum layer convertsincident infra-red wavelengths of 25X 10- cm. into longer Wavelengthinfra-red.

3. A cathode-ray tube according to claim 1, in which said scotophermaterial by itself has very little absorption for infra-red below 25 x10 cm., and said aluminum layer has a thinness which is transparent tored light in the visible range.

4. A cathode-ray tube comprising in combination, an evacuated enclosingenvelope, an electron gun for developing a focussed electron beam, alight-transducing screen comprising a thin backing sheet which is lighttransparent to visible light, a coating of scotophor material on saidsheet facing said gun, a layer of aluminum on said scotophor facing saidgun, an infra-red radiator mounted between said screen and gun, saidaluminum having a thinness which is transparent to said beam and tovisible light and which forms a heat generator in contact with saidscotophor in response to incident infra-red radiation from saidradiator.

S. A cathode-ray tube comprising in combination, an evacuated enclosingenvelope, an electron gun for developing a focussed electron beam, alight-transducing screen comprising a thin backing sheet transparent tovisible light, a coating of scotophor material on said sheet, saidscotophor being normally dark in appearance, a layer of aluminum on saidscotophor having a thinness which is transparent to visible light butwhich is of suflicient thickness to impart a substantial whitishappearance to said screen, and a heatable infra-red radiation generatingfilament mounted between said screen and said gun.

6. A cathode-ray tube according to claim 5, in which said scotophor byitself passes an infra-red radiation of all wavelengths shorter than 25X 10* cm., and said filament is provided with lead-ins for connection toan electric power supply to raise the filament to a temperature at whichit is an efficient radiator of infra-red wavelengths shorter than 25 XlO cm., said aluminum converting said short wavelength infra-redradiation into infra-red radiation of longer wavelength in a range whichis substantially absorbed by said scotophor.

7. A cathode-ray tube according to claim 6, in which said filament is atungsten filament mounted adjacent said screen without materiallymasking the screen against scanning by the electron beam.

8. A cathode-ray tube according to claim 6, in which saicll1 backingsheet has a thickness of the order of 0.001 mc 9. A cathode-ray tubeaccording to claim 6, in which said scotophor and aluminum layer have ananomalous light dispersion surface on the aluminum coated side, and saidaluminum has a critical thinness correlated with the electron velocityof the beam to effect heating of said scotophor in part by anomalousdispersion by the infrared radiation at the surface of the aluminumcoating.

10. The method of making a scotophor screen which comprises depositingscotophor material on a thin heat-insulating but visible-lighttransmitting backing said material having an anomalous dispersionsurface, applying successive coats of vaporized aluminum to saidscotophor until the visual appearance of the screen changes from darkgray to substantially white, and limiting the thickness of said coats tomaintain it transparent to a cathoderay beam while maintaining ananomalous dispersion at said surface.

11. The method according to claim 10, in which the scotophor and saidaluminum coats are applied to said backing in a vacuum.

12. A scotophor screen for dark-trace tubes and the like comprising athin visible-light transparent backing, a coating of scotophor materialon said backing which is substantially transparent to infra-redradiation of wavelengths lower than 25 x 10- cm., and a coating ofaluminum on said scotophor of critical thinness for converting incidentinfra-red radiation of wavelengths shorter than 25 X 10* cm. into aninfra-red radiation of longer wavelength which is substantially absorbedby said scotophor material.

13. A scotophor screen according to claim 12, in which said scotophormaterial by itself has an anomalous dispersion surface and imparts adark appearance to said screen, and said aluminum has a criticalthinness which is transparent to an electron beam but of suflicientthickness to impart a whitish appearance to said screen whilemaintaining said anomalous dispersion surface.

14. A scotophor dark-trace screen according to claim 13, in which saidaluminum comprises successive layers but has an overall criticalthinness whereby the scotophor particles impart an infra-red anomalousdispersion characteristic thereto.

15. A scotophor dark-trace screen according to claim 12, in which saidaluminum has a thinness which is substantially transparent to visiblelight.

References Cited in the file of this patent UNITED STATES PATENTS2,432,908 Leverenz Dec. 16, 1947 2,533,381 Levy Dec. 12, 1950 2,585,846Rosenthal Feb. 12, 1952 2,661,437 Beckers Dec. 1, 1953

1. A CATHODE-RAY TUBE COMPRISING IN COMBINATION, AN ENCLOSING EVACUATEDENVELOPE, MEANS TO DEVELOP A BEAM OF ELECTRONS, A SCREEN UPON WHICH SAIDBEAM IMPINGES TO MAKE A RECORD, SAID SCREEN COMPRISING A LAYER OFSCOTOPHOR MATERIAL WHICH DEVELOPS OPACITY CENTERS WHEN SAID BEAMIMPINGES THEREON, AND MEANS TO ERASE SAID CENTERS, THE LAST-MENTIONEDMEANS INCLUDING A LAYER OF ALUMINUM ON SAID SCOTOPHOR MATERIAL FACINGSAID BEAM-DEVELOPING MEANS, AND AN INFRA-RED RADIATION GENERATOR WITHINSAID ENVELOPE BETWEEN SAID BEAM-DEVELOPING MEANS AND SAID SCREEN, SAIDGENERATOR BEING ESPECIALLY DESIGNED TO PRODUCE SUBSTANTIAL RADIATION ATWAVELENGTHS LESS THAN 25X10-4 CM., SAID ALUMINUM LAYER HAVING A THINNESSWHICH IS TRANSPARENT TO SAID BEAM BUT WHICH DEVELOPS HEAT BY CONVERTINGSAID INFRA-RED RADIATION INTO WAVELENGTHS WHICH ARE SUBSTANTIALLYABSORBED BY SAID SCOTOPHOR MATERIAL.